The International Telegraph and Telephone Consultative Committee (CCITT) has defined the Asynchronous Transfer Mode (ATM) as the transfer mode for implementing Broadband Integrated Services Digital Networks (B-ISDNs). ATM is based on the use of shod, fixed-length packets called cells. A cell consists of an information field prefixed with a header for routing and control information. The ATM cell structure, having a header portion being 5 octets long and an information field being 48 octets long, is described in "A Direct ATM Connectionless Service", J. Y. Le Boudec et al.. RACE 1035 Temporary Working Document WP3.2/IBM/030.1, 23 Aug. 1991, pp. 1-15.
ATM cells are routed based on the contents of the Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) fields in the header of said cell. The VPI/VCI values are used in intermediate switching nodes in conjunction with routing tables to determine the outgoing link on which the cell should be forwarded. They are only valid for the current link and are in general replaced by a new value at the next section. The routing table in any switching node contains an entry for each VPI/VCI value assigned on each incoming link, with the entry providing a mapping to the appropriate outgoing link and the new VPI/VCI value to be used on that link. The assignment of the VPI/VCI values and construction of the routing table entries are carried out as part of a connection setup procedure. Hence, ATM networks provide basically a Connection-Oriented (CO) virtual circuit service.
Different aspects of ATM and its application are described in the following publications:
CCITT Study Group XVIII, Report R34, June 1990, including: PA0 "Principles and Benefits of the Asynchronous Transfer Mode", Dupraz et al., Electrical Communication, Vol. 64, No. 2/3, 1990, pp. 116-123; PA0 "The Virtual Path Identifier and Its Application for Routing and Priority of Connectionless and Connection-Oriented Services", J. L. Adams, Int. J. of Digital and Analog Cabled Systems, Vol. 1, No. 4, 1988, pp. 257-262.
Draft Recommendation I.121: Broadband aspects of ISDN; PA1 Draft Recommendation I.150: B-ISDN ATM functional characteristics; PA1 Draft Recommendation I.311: B-ISDN general network aspects;
A method for supporting the Asynchronous Transfer Mode ConnectionLess (ATM CL) service is disclosed in the European Patent Application EP 91810669.1 with title "Connectionless ATM Data Services" and in its counterpart U.S. application Ser. No. 07/807,359 filed Dec. 16, 1992. In addition, this method is reported on in the working document "A Direct ATM Connectionless Service", J. Y. Le Boudec et al., RACE 1035 Temporary Working Document WP3.2/IBM/030.1, 23, Aug. 1991, pp. 1-15. In this method, every participating terminal (TE) is assigned a Domain Address (D@). The D@ of the destination TE is placed in the VPI/VCI field of the ATM cells by the source TE. The ATM network will then route the ConnectionLess (CL) cells to the destination, based on the Domain Address (D@). For this purpose the D@ is divided into two parts, one for identifying the switching node and the other one for identifying the switching node access port to which the destination TE is attached.
A disadvantage of this ATM CL service is that the Domain Address concept can only be applied to domains with a maximum number of active TEs limited by the size of the D@. This maximum number is namely ca. 64000. Networks with a larger number of terminals have to be partitioned into multiple domains and an additional mechanism is needed for routing the CL cells between these domains.